What is causing climate change?
Geological records stretching back millions of years indicate a number of large variations in Earth’s climate. These have been caused by many natural factors, including changes in the sun, volcanoes, Earth’s orbit and CO2 levels.
However, research by the Intergovernmental Panel on Climate Change (IPCC) shows that it is 90% likely that human activity has caused more recent global warming.
Greenhouse gas emissions
Evidence that CO2 emissions are the cause of global warming is very robust. Scientists have known since the early 1800s that gases in the atmosphere trap heat.
Global CO2 emissions from human activity have increased by over 400% since 1950. As a result, the concentration of CO2 in the air has reached more than 400 parts per million by volume (ppm), compared to about 280ppm in pre-industrial times (pre-1750).
The Earth’s natural climate cycle
Over the last 800,000 years, there have been natural cycles in the Earth’s climate. There have been ice ages and warmer interglacial periods. After the last ice age 20,000 years ago, average global temperature rose by about 3°C to 8°C, over a period of about 10,000 years.
We can link the rises in temperature over the last 200 years to rises in atmospheric CO2 levels. Rises in temperature are now well above the natural cycle of the last 800,000 years.
The sun is the primary source of Earth’s heat, so relatively small changes in solar output can affect our climate.
Satellite observations since the late 1970s have shown a slight decrease in the sun’s total energy output. However, instead of cooling, the Earth has warmed over this period.
Also, warming from the sun would heat all of the atmosphere, including the lowest few kilometres (the troposphere) and the layer above (the stratosphere). Observations show that the stratosphere is in fact cooling while the troposphere warms. This is consistent with greenhouse gas heating and not solar heating.
Trends in stratospheric temperature since systematic measurements began, from eight different datasets.